As waste material decomposes in a landfill, it gives off various gases. In the past, it has been known to use pumps, piping, and wellheads to extract the gases from the landfill and collect the same. Such wellheads are often spaced about one per acre in a grid pattern. Such systems of collecting the gases can be shut down by many factors, including power failures. To prevent the undesirable build-up of such gases in the event of non-operation of the extraction system, it has often been known to employ a grid pattern of vents spaced between the extraction wellheads, often at the same one per acre density.
As described in published U.S. Patent Application Number 20060034664, conventional gas extraction wells at landfills often involve deep wells attached to a network of pipes and a gas pump (blower) that applies vacuum (negative pressure) to extract the gas from the stored waste as the waste decomposes. The profile of surface emission flux is recognized to lead to potential for some emissions away from the wells under many circumstances. Note also that there is almost always entrainment of gas, whether LFG or atmospheric air, through the surface area most proximate to deep collection. Both LFG emission far from wells, and air entrainment proximate to subsurface collection, are well recognized as deleterious to collection efficiency. A “tradeoff” exists between extracting or “pulling” at too high a flow rate and entraining excessive atmospheric air, and pulling too little and recovering less LFG. This poses one dilemma of conventional extraction.
A prior art arrangement according to the above published patent application is shown in FIG. 1. Landfill 1 containing waste 2 generates biogas (biogas flows shown by the arrows). Biogas is collected and extracted through a well 3. The well 3 includes a gas-collecting well screen 16 and a gas-impermeable conduit 17 linking the well screen to the surface to draw biogas from the wellhead to the surface. Overlaying the majority of the waste 2 is a gas-permeable layer 5. The term “wellhead” refers to a portion of the gas-extraction well from which gas can be extracted. The well often includes a section of pipe having slots or other gas-flow apertures cut in it, referred to as a “well screen”. Often, the well screen is also surrounded with gravel. The gas-permeable layer is typically composed of a conductive porous matrix with gas flow paths. Often it is composed of rigid or semi-rigid particles of a large enough size to leave a significant void volume between particles. For instance, the gas-permeable layer may contain sand, gravel, wood chips, or shredded tires. Above the gas-permeable layer is a gas-containment layer 7. Biogas that rises from the landfill reaches the gas-permeable layer where it is trapped by the overlying gas-containment layer 7. The biogas migrates horizontally in the gas-permeable layer until it comes close to a well. Gas extraction from the well creates a vacuum that draws gas into the well. This vacuum draws biogas from the overlying gas-permeable layer down through the waste mass of the landfill to reach the well. The area immediately beneath the gas-permeable high conductivity layer 5 through which a substantial fraction of the biogas from the gas-permeable layer passes as it travels to the gas-collection wellhead is the entrainment zone 9. On its passage through the waste 2, the gas from the gas-permeable layer mixes with biogas produced in the waste mass that has not gone through the gas-permeable layer. This helps to give a consistent content to the biogas that is withdrawn from the well. If gas is withdrawn directly from the gas-permeable conductive layer, the gas composition will vary more dramatically over time, sometimes containing a high air content and sometimes not. It is sometimes desirable to place an even more impermeable layer, such as geomembrane 15, directly over the zone of entrainment of gas from the permeable layer that is created by the deep well. Moreover, sometimes the entire landfill is covered with such a membrane.
When such gas extraction from landfills is interrupted, such as by power outages or equipment failure, it is important to continue to remove the gas from the landfill, to prevent the gas from building up and destabilizing the covering. As described above, the use of passive vents has been common. Such passive vents often take the form of a pipe stuck in the ground with a turned-down upper end to prevent rain from entering the vent. Sometimes such vents have valves in them, other times not. In any event, ideally such a vent should work automatically, whether there is available electric power or not, and be mechanically robust and reliable.
Accordingly, it can be seen that there exists a need for a better way for venting sub-surface gas from near the surface of landfills. It is to the provision of solutions to this and other problems that the present invention is primarily directed.